I really like the mechanism of the Impossible?? to open gift box by user 3d_Matthias at Thingiverse. However, I had different magnets at hand and needed a box of different size.

I made this model from scratch, but since it's basically a copy of 3d_Matthias' design I still mark it as a remix.

Besides the printed parts this model requires a 10mm steel ball and a 10mm x 2mm disc magnet.

How the box works

While the outside of the box looks like it's interlocked by dove tails that seem impossible to disassemble, you can in fact slide it open diagonally. But there's another catch: there's a steel ball held in place by a magnet right in the sheer line between the halves, which locks them in place. To unlock the box you need to slam it into your hand which detaches the ball from the magnet giving you the chance to slide the box open.

Printing

• Supports: none
• Layer height: 0.1mm for a nice finish of the side walls, 0.2mm first layer
• Perimeters: 2-3
• Other: Use ironing on the contact surfaces. This makes sliding much smoother. You can either enable it on all top surfaces or use a height range modifier to enable it only on the contact surfaces to save some time.

Tuning

Since your magnet's strength might vary, you should tune how hard you have to hit the box. Print the ‘TuningTube’ (doesn't need to be pretty, use 0.2mm layer height to save time) along with different height cylinders of 10mm diameter. I suggest trying 1, 2 and 3mm. You can easily create these in PrusaSlicer or its forks (SuperSlicer, OrcaSlicer or BambuStudio).

Insert the magnet into the tube from the top and the steel ball from the bottom, without a spacer disk at first. 

Hit the tube into your hand to try and get the steel ball out.

Now remove the magnet, insert one of the spacers before the magnet and repeat the above. Also try the other spacers until you're satisfied with how much force it takes to get the steel ball out.

Final Assembly

With your chosen spacer you need to do a little math: To place the ball exactly at the sheer line, you need to glue a 7mm tall stack into the hole of the lid.

Example: spacer is 2mm, magnet is 2mm. So you need to put another 3mm spacer below the magnet.

Glue the stack in place and let it cure. 

Finally, insert the steel ball into the bottom hole, slide the lid on and turn the whole box around once to get the steel ball in place.